The present invention relates to a method of and an apparatus for driving lighting devices utilizing cathodeluminescence. More particularly, the invention is directed to a method of and a circuit for driving a lighting device suitable for use as a light source for an image input apparatus such as an image scanner, a digital copying machine, or a facsimile machine, or for a large display such as an outdoor television-type display.
In lieu of conventional light sources such as fluorescent lamps and line halogen lamps used as a line light source in an image input apparatus or the like, a light source utilizing cathodeluminescence, such as disclosed in Japanese Patent Unexamined Publication No. 73970/1981, has been proposed as a light source having low power consumption.
FIG. 1 is a sectional view of a conventional cathodeluminescence type light source. The construction of the conventional light source will be described with reference to FIG. 1.
Reference numeral 10 designates a glass tube that is evacuated to a predetermined degree of vacuum. Along the length of the glass tube 10 extends a cathode 12. Reference numeral 14 designates a glass substrate, a front surface of which is coated with a phosphor 16. An anode 18 is deposited on the phosphor 16. The anode 18 is made of an aluminum thin film that serves as a metal backing. A grid 20, which is a control electrode, is interposed between the cathode 12 and the anode 18, and is made of a metal plate having slits 11 of a length of about 0.3 to 1 mm. Commonly used as the phosphor materials are those including zinc sulfide (ZnS) as a base material, such as ZnS with copper (Cu) and aluminum (Al). The light-emitting efficiency of these phosphor materials is high.
A high voltage of about 8 kV is applied to the anode 18 of the light-emitting tube by an ordinary power supply E2, whereas a voltage V4 on the order of several volts is applied to the grid 20 by a power supply E4.
A driving method will be described next. Japanese Patent Unexamined Publication No. 3740/1978 discloses a method of driving the cathodeluminescence type light source. FIG. 2 is a timing chart for driving such light source. CTLon designates a lighting signal input to a cathode drive circuit (not shown). Cathode drive signals Ek1 and Ek2, which are output signals generated from the cathode drive circuit, are applied to the terminals K1 and K2 of the cathode 12 of the light-emitting tube shown in FIG. 1. The generation of the lighting signal CTLon is repeated with a cycle time t.sub.0 based on a turning-off period t.sub.off and a lighting period t.sub.on.
During the turning-off period t.sub.off, the voltages of the cathode drive signals Ek1 and Ek2 are V2 and V1, respectively. The voltages V2 and V1 are higher than the voltage V4 applied to the grid 20. When the voltages V2 and V1 are applied to the terminals K1 and K2 of the cathode 12, a difference in voltage .DELTA.V=V2-V1 is supplied to the cathode 12 to cause the surface of the cathode to easily release thermoelectrons 24 (described later) by Joule heating.
However, since the voltages V2 and V1 applied to the terminals K1 and K2 of the cathode 12 are higher than the voltage V4 applied to the grid 20, there is no emission of thermoelectrons 24 from the cathode 12.
During the lighting period t.sub.on, the voltages of the cathode drive signals Ek1 and Ek2 both are 0 V. Since the voltage of the cathode 12 is lower than the voltage V4 applied to the grid 20, the cathode 12 is heated by remaining heat, releasing thermoelectrons 24.
The thermoelectrons 24 pass through the slits 11 of the grid 20 and collide against the anode 18, to which the high voltage is applied. The thermoelectrons 24, having passed through the anode 18 formed of a thin film, collide against the phosphor 16, causing cathodeluminescence to occur.
The rate at which thermoelectrons are emitted from the surface of the cathode 12 depends on the surface temperature of the cathode 12. When a thermoelectron emitting substance called an emitter material (not shown) is coated on the surface of the cathode 12, thermoelectrons are usually emitted therefrom at an adequate rate at surface temperatures of 500.degree. to 700.degree.. However, if the temperature is too high, evaporation of the emitter material is promoted, reducing the rate of emission of thermoelectrons. As a result, the amount of light emitted from the light source is impaired drastically. On the other hand, if the temperature is too low, the emission of thermoelectrons is not sufficient. To overcome this problem, three points, namely, the conduction voltage, the cathode resistance, and the conduction duty cycle at the time of operation (i.e., the ratio of the off period t.sub.off to the lighting cycle t.sub.0 in FIG. 2) relevant to heating the cathode must be properly selected before driving the cathode.
However, in a lighting device used for an image input apparatus or the like, the integrated value of an amount of light stored in a CCD of a photoelectric transducer of the apparatus is made adjustable by making the lighting period t.sub.on of the lighting device variable to increase the accuracy of the output signal from the CCD. However, if the lighting period t.sub.on is varied in the conventional drive circuit, the off period t.sub.off must also be varied (assuming the lighting cycle time t.sub.0 is fixed). That is, the conduction duty cycle must be varied, which puts the surface temperature of the cathode 12 out of proper range, thus shortening the life of the light-emitting tube.